I believe its experimental weaponry & aircraft/vehicles

They've hid it from us this long, and they have no intentions of telling us in our lifetime. I honestly think if it ever got leaked and what got leaked was true, It would change humanity forever..

Experimental aircraft. Nothing more.
It would be a shit place to hide aliens or anything that interesting.

Lots of donuts. Maybe some shitty coffee.

The krabby patty secret formula

Experimental technology, not just Aircraft and Vehicles but the future of computing hardware, weaponary and likely human ability (Develop mental capacity and ability perhaps)

They probably keep UFO related things there too, obviously not UFO as in a flying saucer with a little green fella but objects from Space, the Moon etc.. which have yet to be identified or are being deeply analysed.

The question is, if there's so much conspiracy about a place we know very well, it's location and size.. What could there be at places we don't know about?

Like some mentioned, I think they are experimenting with future weaponry and vehicles, aircraft etc. I think they may have some classified information in there as well, gathered over the past many years.

Human experiments and weaponry

I just believe the USA is experimenting with new technology such as new types of Weapons and Vehicles to be ahead for war. They won't let anybody in because they don't want people stealing ideas. Just my thought, I don't know why they would hold aliens back there doesn't even seem reasonable

I think that they hide some shit in there. It would be crazy to find out I imagine.